Argus: Introduction

Argus Introduction

The Argus authorization service is designed to answer questions in the form of Can user X perform action Y on resource Z at this time? Not surprisingly, two pieces of information are required to answer this question; the request that describes X, Y, and Z and the policy against which the request is evaluated. The purpose of this introduction is to provide an understanding of the logical contents of the request and the policy. Such an understanding will help in creating appropriate access control policies for a service. This introduction does not cover the command line tools, simple policy language, or underlying XACML policies used by the authorization service. That information can be found in the Policy Administration Point documentation.

Attribute Based Descriptions

Before discussing the request and response it is important to understand how users, actions, and resources are identified. Most grid deployers will be familiar with X.509 subject distinguished names (DNs) based access control. Access control systems which render authorization decisions based only on an identifier, like a subject DN, are known as identity-based access control systems. However, an application usually knows more about a user than simply their identifier. It may know for which organization the user works, or groups and professional organizations to which the user belongs. An access control system which can render a decision based on a set of attributes describing the users/resources/actions is known as an attribute-based access control system. Note, an attribute-based access control system where policies are only based on an identifier attribute is functionally equivalent to an identity-based access control system.

Argus is an attribute-based system. It uses attributes to identify the user attempting to perform an action, the resource on which the action is to be performed, the action itself, and other environmental information. Within the system an attribute is made of 4 pieces of information: a unique identifier/name, one or more values for the attribute, the data type of the attribute values (e.g. a string, an integer, an email address), and an optional identifier for the issuer of the attribute. The following are examples of attribute based descriptions.

In an identity based system a user (more properly known as a subject) might have the identity 'jsmith'. In an attribute based system the subject might be identified by the following attributes:

  • id: subject-id, datatype: string, value: jsmith
  • id: org, datatype: string, value: CERN
  • id: affiliation, datatype: string, value: employee
  • id: vo, datatype: string, value: lhcb, cms, atlas

In another example an action in an identity based access control system might be identified by the string 'submit-job'. In an attribute based system the action might be identified with the following attributes:

  • id: action-id, datatype: string, value: submit-job
  • id: pilot-job, datatype: boolean, value: false
  • id: executable, datatype: string, value: /usr/bin/myexec
  • id: expected-execution-duration, datatype: integer, value: 10

The exact attributes available within a request are determined by the application seeking an authorization decision. Obviously each unique application will have different actions it wishes to protect, a different means of describing itself, and possibly different information about the subject. Also, as can be seen by these examples, the amount of information upon which a decision could be based is larger within an attribute based access control system than an identity based system. As demonstrated, the single piece of identity data used in an identity based system can also be used within an attribute based one.

The Request

An authorization decision request is simply a set of four different attributes collections. The four collections are:
  • subject - contains attribute describing user who is trying to perform an action
  • action - contains attributes describing the action the subject is attempting to perform
  • resource - contains attributes describing the program within which the subject is attempting to perform the action
  • environment - contains attributes relevant to the decision but not part of the previous three collection (e.g. time/date of the request, machine used by the subject, etc.).

A request must contain at least one attribute in each of the subject, action, and resource sets but it is common for there to be no environment attributes.

The Policy

A policy is a collection of rules that are evaluated to determine the result of an authorization request. The result of a policy evaluation may be:
  • permit - indicates the subject is permitted to perform the action on the resource
  • deny - indicates the subject is not permitted to perform the action on the resource
  • not applicable - indicates no policy applied to the request and so no decision could be reached
  • indeterminate - indicates there was an error evaluating the policy

In order to determine whether a rule is met each rule contains a target. The target defines one or more combination of attribute which trigger the rule. When more than one combination is listed any one may trigger the rule. For example, the target of a rule may stipulate the following combination:

  • subject attribute vo has a value of atlas and the id of the action is job-submit and the id of the resource is cern-ce
  • subject attribute id has a value of john and the id of the action is job-submit and the id of the resource is cern-ce
  • subject attribute id has a value of jane and the id of the action is job-submit and the id of the resource is cern-ce

As you can see, each combination of attributes within the target needs to be complete (that is they need to describe the whole situation that would trigger a rule), there is no mechanism for expressing logical operations like AND, OR, NOT. As you can imagine this could lead to a lot of repeated information. To help alleviate this, policies, the container of rules, can also contain targets. Therefore the previous rule target could be written as follows:

  • A policy with target: the id of the action is job-submit and the id of the resource is cern-ce
  • A permit rule with the target combination:
    • subject attribute vo has a value of atlas
    • subject attribute id has a value of john
    • subject attribute id has a value of jane

You can also have multiple rules within a single policy, for example:

  • A policy with target: the id of the action is job-submit and the id of the resource is cern-ce
  • A permit rule with the target combinations:
    • subject attribute vo has a value of atlas
    • subject attribute id has a value of john
    • subject attribute id has a value of jane
  • A deny rule with the target combinations:
    • subject attribute id has a value of christoph

In fact, policies can even contain other policies which helps in the case where you may want a policy about a resource which in turn contains a policy about actions (at that resource) and finally rules for that action. For example:

  • A policy with target: id of the resource is cern-ce contains:
    • A policy with target: the id of the action is job-submit
    • A permit rule with the target combination:
      • subject attribute vo has a value of atlas
    • A deny rule with the target combination:
      • subject attribute id has a value of christoph
    • A policy with target: the id of the action is job-manage
    • A permit rule with the target combination:
      • subject attribute id has a value of john
      • subject attribute id has a value of jane

Not Applicable and Indeterminate Results

In some cases, either because no policy applied to the request or because there was an internal error evaluating a policy, a decision other than permit or deny is returned. In the event that a result of not applicable or indeterminate is returned Argus will treat this as a deny. Thus Argus is said to be a deny biased system.

Example Requests and Policies

More complete request/policy examples can be found here.

Identifiers within Argus

Argus makes use of various identifiers for attribute identifiers and values. The identifiers must by URIs (URNs or URLs). The responsibility for defining such identifiers falls to different individuals, depending on the expected scope of use of the identifiers.

Entity ID

For those familiar with other grid services one of the first differences that you will encounter when setting up Argus is the requirement to provide entity IDs for the service components. These IDs provide a way of uniquely identify a logical service component. We use the term "logical service component" because each service component may be clustered. So the logical instance is the set containing all the physical instances participating in the cluster.

It is the deployer of service component that determines this identifier. The identifier may be any URI for which the deployer is authoritative. That is, it must come from a domain (in the case of a URL) or namespace (in the case of a URN) that the deployer controls. The following formula is a reasonable means for generating these identifiers: http://{authz_domain}/{service_component_identifier}. The domain should be a domain name that is not linked to any particular servers hostname but is instead related to the service (e.g. authz.example.org). The component identifier should be pap, pdp, or pepd depending on which component is being installed.

If an organization runs more than one component an additional qualifier may be added to appended to the path of the URL identifier. For example an organization that runs a different PEPd for each computer cluster they operate might choose the entity IDs http://authz.example.org/pepd/cluster1 and http://authz.example.org/pepd/cluster2

Resource ID

The resource identifier used to identify the resource being protected by the authorization service (e.g. a compute cluster, a portal). This identifier is specific to a given instance of the resource. If an organization runs two different portals each portal receives a different ID. These identifiers may then be used within a policy in order to indicate policies that apply to the specific resource (i.e. policy A applies to portal 1 and policy B applies to portal 2).

The resource identifier is selected by the deployer of the resource. Two reasonable ways for generating this identifier are:

  • if the resource has a natural URI identifier associated with it (e.g. the main page of the portal) that may be used
  • if the resource does not have a natural URI identifier a synthetic one may be created with the following formula: http://{authz_domain}/resource/{resource_identifier} The authz_domain should be a domain name that is not linked to any particular server's hostname but is instead related to the authorization service (e.g. authz.example.org). The resource identifier is simply a unique string for the protected resource. A human intelligible string is best (e.g. sequencingPortal).

Action ID

The action identifier is used to identify the action for which the resource is requesting authorization. This action identifier is specific to a given piece of software but all instances of that application use the same identifier for a given action, it is not deployment specific.

The action identifier is selected by the developer of the application. The identifier may be generated in one of two ways:

  • If the application is widely used a request could be made to a standards body, such as IANA, for an unique identifier for each action of the application. This way all implementations of the specification will likely use the same identifier and thus make it a bit easier for policy writers.
  • The application developer may generate an identifier. A reasonable formula for such an identifier is: http://{application_domain_name}/{application_name}/action/{action_id} where the domain name is the domain component of the application's website and the action ID is a human readable string for the action (e.g. =readFile, addUser)

Attribute ID

The attributes produced by an application identify bits of information that the application was able to gather and make available for an authorization request. The identifiers are specific to a given piece of software but all instances of that application use the same identifier for a given attribute, it is not deployment specific.

The attribute identifier is selected by the developer of the application. The identifier may be generated in a couple of ways:

  • If the attribute is already defined by an existing standard (the LDAP schema standards are a great place to start looking) the identifier from that standard may be used. In the case of the LDAP schema the URN urn:oid:{ldap_attribute_oid} can be used.
  • If the attribute is likely to be widely used a request could be made to a standards body, such as IANA, for an unique identifier for this attribute. This way all implementations of the specification will likely use the same identifier and thus make it a bit easier for policy writers.
  • The application developer may generate an identifier. A reasonable formula for such an identifier is: http://{application_domain_name}/{application_name}/attribute/{attribute_id} where the domain name is the domain component of the application's website and the attribute ID is a human readable string for the attribute (e.g. username, entitlements)

In general, application developers should prefer already defined attributes over creating their own.

Conclusion

At this point you should understand what an attribute is, that a request is a made of subject, action, resource, and environment attributes, what a rule is and that policies are a collection of rules. You should also understand that a policy or rule is triggered if any one of the combination of attribute/values listed within its target is present within the request.
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Topic revision: r9 - 2016-07-05 - MaartenLitmaath
 
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